So far, I've only put leds in my ZZ. That required 3mm round leds to prevent interference with steering and other components. The ZZ has more room than a Bit. Test fitting 5mm leds in my Bit (and the ZZ), I found they were way too big. I'd imagine 3mm round units will work just fine in the Bit too though.
As for sources, I'm not too sure where to find them. I think I saw a listing in another thread. I've heard
www.eled.com mentioned. Their prices seemed a little high though.
www.partsexpress.com has much better pricing, but a more limited selection.